Development of optical sensors for hydrogen environment applications

Hydrogen is regarded as a clean and sustainable energy carrier and has attracted much attention in recent years as a power resource in order to lower the CO2 emission. The hydrogen-based power resource is identified as a sustainable alternative – especially in the industrial production, infrastructure, and mobility sectors. However, hydrogen is used, appropriate safety precautions must be considered. If the hydrogen concentration in the air exceeds a threshold of 4%, a small ignition source like a single spark could be enough to trigger an explosion. For that reason, monitoring of hydrogen infrastructure with sensors is mandatory. Optical fiber sensors are considered suitable for this role due to their safety advantages and immunity to electromagnetic interference. In contrast to electric sensors, no wires are involved that could potentially spark and ignite the gas mixture.
In this dissertation, in optical sensors are designed and characterized for hydrogen
environment applications, with a focus on both optical hydrogen leak detection and
hydrogen-resistant sensing applications for hydrogen-rich atmospheres technologies. The
sensors presented in this work are based on fiber Bragg gratings (FBGs) manufactured using femtosecond laser technology via the point-by-point technique.
One type of sensor developed is an FBG coated with a palladium-silver layer, upon
interaction of hydrogen a stress-induced signal change arises. Various coating thicknesses were tested, showing that a thicker PdAg layer leads to greater wavelength shifts. The sensor demonstrated high reproducibility and was able to detect hydrogen concentrations as low as 0.1%.
Another hydrogen detection sensor, which exhibited significant potential for hydrogen
detection, is an FBG coated with palladium nanoparticles that shows signal change due to interaction within the evanescent field. This sensor consistently reproduced changes in intensity across multiple hydrogen exposure cycles, demonstrating stability and reliability. At room temperature, it has a dynamic working range from the detection limit (0.3%) up to 5%, addressing most applications requiring early alarm management for safety. The sensor also features a short response time (under 90 seconds) and recovery time (under 140 seconds), with no observed hysteresis, making it ideal for practical applications. To address hydrogen-resistant sensing, a successful inscription and characterization of FBGs using femtosecond laser pulses at 400 nm in carbon-coated optical glass fibers is presented. These fibers are designed for enhanced hydrogen resistance. The FBGs exhibited low polarization dependence. After 30 days of exposure to 100% hydrogen at 84°C, the FBGs maintained stable reflection spectra, with minimal changes in wavelength and intensity.
Furthermore, the optical hydrogen detection sensors were applied in a real-life scenario to detect hydrogen venting during the failure of lithium-ion batteries (LIBs). This application aimed to provide early detection of thermal runaway to mitigate its propagation in LIB storage facilities.